1. Field of the Invention
The present invention relates to thin film transistor (TFT) liquid crystal display (LCD) structures, and, in particular, to a TFT LCD incorporating an LED structure for backlighting.
2. Description of the Related Art
A thin film transistor liquid crystal display is a variant of a liquid crystal display (LCD) which uses thin film transistor (TFT) technology. A TFT LCD is one type of active matrix LCD, and TFT LCDs are used in many different types of consumer electronic devices, such as television sets, computer monitors, mobile phonies, handheld video game systems, personal digital assistants, navigation systems, and projectors. In TFT LCDs, pixels are addressed in rows and columns, and each pixel is provided with its own transistor switch that allows each pixel to be individually controlled. The low leakage current of the transistor prevents the voltage applied to the pixel from leaking away between refreshes to the display image. Each pixel is a small capacitor with a layer of insulating liquid crystal sandwiched between transparent conductive Indium Tin Oxide (ITO) layers.
The circuit layout of a TFT LCD is very similar to that of a DRAM memory. However, rather than fabricating the transistors from silicon formed into a crystalline wafer, they are made from a thin film of silicon deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel; the rest of the silicon film is etched away to allow light to pass through. The silicon layer for TFT LCDs is typically deposited using the Plasma Enhanced Chemical Vapor Deposition (PE CVD) process from a silane gas precursor to produce an amorphous silicon film. Polycrystalline silicon is sometimes used in displays requiring higher TFT performance. Examples include high-resolution displays, high-frequency displays or displays where performing some data processing on the display itself is desirable. Amorphous silicon-based TFTs have the lowest performance, polycrystalline silicon TFTs have higher performance (notably mobility), and single-crystal silicon transistors have the best performance.
FIG. 1 shows a top view of a conventional prior art LCD array 101 employed for a TFT LCD display. Each square, such as square 102, represents a TFT LCD cell with its associated charging capacitor, switching transistor, and biasing elements. In addition, a large number of TFT LCD displays employ backlighting. Backlighting provides a light source to provide brightness from the rear of the display panel, which light source generally includes an array of individual light sources dispersed through arrays of the panel. In some applications, backlighting might be accomplished through use of an array of light emitting diodes (LEDs), but in most applications backlighting is accomplished through an array of cold cathode fluorescent lights (CCFLs). CCFLs require high voltages to fluoresce, which voltages are achieved using switching boost regulators. Consequently, FIG. 1 also shows CCFL arrays 103 and 104, illustrating placement of CCFLs, such as CCFL 105, along the edges of a TFT LCD array to provide backlighting. FIG. 2 shows an edge view of the TFT LCD array of FIG. 1. As shown) in FIG. 2, Si substrate 201 includes array 202 of TFT LCD cells 203(1) through 203(N) grown on substrate 201 by known techniques. Above Si substrate 201 is liquid crystal (LC) layer 204 including a twisted nematic (TN) layer, polarizer (electrode) and color filter. LC layer 204 is covered by glass panel 205. Dashed lines in FIG. 2 are meant to show regions of the device corresponding to TFT LCD cells 203(1) through 203(N) when formed during the manufacturing process by patterning electrodes above and below LC layer 204.
The inexpensive TN display is the most common display type used in consumer electronics. The TN effect is based on the precisely controlled realignment of liquid crystal molecules between different ordered molecular configurations under the action of an applied electric field. A twisted configuration of nematic liquid crystal molecules (e.g., within LC layer 204) is formed between two glass plates (Si substrate 201 and glass panel 205), which are separated by several spacers and coated with transparent electrodes. The electrodes themselves are coated with alignment layers that precisely twist the liquid crystal 90° when no external field is present. When light is incident on one side of the panel, which behaves as transparent to the wavelengths of light concerned, the light having proper polarization passes through the first polarizer and into the crystal. Light in the crystal is rotated by the helical structure. The light is then properly polarized to pass through the second polarizer, set at 90° to the first. The rotated light then passes through the other side of the LCD cell, which also behaves as transparent to the wavelengths of the light concerned.
When a field is applied between the two electrodes, the crystal re-aligns itself with the external field, breaking the twist in the crystal, Without the twist, the polarized light passing through the crystal fails to re-orient. In this case the light is blocked by the real polarizer, and the cell becomes opaque. The amount of opacity is controlled by varying the voltage applied between the two electrodes; at voltages near a threshold only some of the crystals will re-align, and the display will be partially transparent, but as the voltage is increased more and more of the crystals will re-align until it becomes completely switched. To display information with a twisted nematic liquid crystal, the transparent electrodes are structured by photo-lithography to form a matrix or other pattern of electrodes. Only one of the electrodes has to be patterned in this way, the other can remain continuous.